I. Field of the Invention
The present invention relates to high-density resin tablets for encapsulating a semiconductor device, a method for manufacturing such tablets, and a method for encapsulating a semiconductor device using such tablets.
II. Description of the Prior Art
At present, transfer molding is widely adopted for plastic encapsulation of semiconductor devices. In transfer molding, a metal lower mold die is used which has a number of cavities and a concave portion called pot communicating therewith through runners. A lead frame at the outer periphery of a semiconductor device is placed on an edge of the cavity. An upper mold die having cavities and a through hole corresponding to the cavities and the pot, respectively, of the lower mold die is placed on the lower mold die. Thus, a semiconductor device is arranged in each space defined between corresponding cavities of the lower and upper mold dies. When a thermosetting resin is heated, it is generally plasticized and is thereafter cured. Accordingly, if a thermosetting resin tablet which is preheated by a high-frequency heater or the like is inserted in the pot of the mold die which is preheated to a predetermined temperature, the resin tablet in the pot is plasticized. When the plasticized thermosetting resin is fed out from the pot by means of a plunger, the resin reaches the cavities through the runners and covers the semiconductor devices in the cavities. When the resin is left to stand for a while it cures, thus encapsulating the semiconductor devices.
In the transfer molding method, however, the resin remained in the pot and the runners is discarded, so that the utilization efficiency is bad. To eliminate this disadvantage and to enable faster molding, multitablet molding method has been developed and used. This method also uses lower and upper mold dies. The lower mold die has a plurality of pots. In the vicinity of the each pot, a pair of cavities are formed such that they face each other with the pot between. The pair of cavities are communicated with the pot through thin and short grooves called gates. A lead frame of a semiconductor device is placed on the edge of each cavity. An upper mold die having cavities and through holes corresponding to the cavities and the pots, respectively, of the lower mold die is placed on the lower mold die. Thus, a semiconductor device is arranged in each space defined between corresponding cavities of the lower and upper mold dies. A thermosetting resin tablet which is not preheated is inserted into each pot through the through hole and the resin tablet is plasticized therein. After the resin is plasticized, it is pressed by a plunger, so that it is injected into the cavities with pressure. The resin is cured in the cavities and thus the semiconductor device is encapsulated. The pots of the lower mold die used in the multitablet molding method is smaller than that of in the transfer molding method. The distance between the pot and the cavity is shorter in the multitablet molding. As a result, the amount of discarded resin is smaller and the injection time is shorter in the multitablet molding. Further, in the multitablet molding, a fast-curing resin may be used so that the quicker molding may be attained when compared with the transfer molding method.
In conventional molding, a thermosetting resin is formed into tablets by a tableting machine so as to allow easy handling of the resin during encapsulation or transportation. Thus, the resin need only be formed into tablets which have a hardness which does not allow chipping or cracking. Accordingly, the resin is conventionally formed into tablets at a pressure of about 0.5 to 1.0 ton/cm.sup.2. The compressibility of the obtained resin tablets is about 77 to 84%, where the compressibility is given by
Compressibility (%)=tablet density/density of molded product
Since the density of the molded product remains the same for each type of thermosetting resin, the tablet density increases with an increase in the compressibility. In other words, the higher the compressibility, the greater force the resin is pressed together.
However, when the cross-section of a molded product obtained by conventional molding is examined, inner voids of about 0.1 mm size can be detected. Some have several voids of 0.5 to 1.0 mm. The occurrence of the voids is severer in the multitablet molding method in which fast-curing resin is usually used. The outer surface of a molded product also has voids. Especially, in the multitablet molding method, an outer void called "snake eye" is often appeared in the vicinity of the gate of the mold die. Such a void not only degrades the outer appearance of the device, but also the reliability of the resin-encapsulated semiconductor device, especially its resistance to thermal shock and to humidity. When the number of inner voids increases, disconnection of the bonding wire connecting the lead frame of the device and pellet elements occurs more frequently.